1. Field of the Invention
The process of this invention provides for reforming of a hydrocarbon stream having a limited C.sub.9 + hydrocarbons content. The improved process is beneficial for any of several purposes, including the upgrading of motor gas (mogas) pools, or enhancing the yield of aromatic compounds in petrochemical operations.
2. Description of Material Information
Hydrocarbons can be subjected to a variety of processes, depending upon the product or products desired, and their intended purposes. A particularly significant process for treating hydrocarbons is that of reforming.
In hydrocarbon conversion, the reforming process is generally applied to fractions in the C.sub.6 -C.sub.11 range. The light fractions are unsuitable because they crack to lighter gases at reforming conditions; the heavier fractions cause higher coking rates (deposition of carbon on the catalyst), and therefore accelerate deactivation of the catalyst.
A variety of reactions occur as part of the reforming process. Among such reactions are dehydrogenation, isomerization, and hydrocracking. The dehydrogenation reactions typically include dehydroisomerization of alkylcyclopentanes to aromatics, dehydrogenation of paraffins to olefins, dehydrogenation of cyclohexanes to aromatics, and dehydrocyclization of paraffins and olefins to aromatics. Reforming processes are especially useful in petrochemical operations for upgrading mogas pool octane value, and in petrochemical operations for enhancing aromatics yield.
Different types of catalysts are used for conducting the reforming of hydrocarbon streams. One means of categorizing the type of catalysts so used is by designating them as "monofunctional" and "bifunctional" catalysts.
Monofunctional catalysts are those which accomplish all of the reforming reactions on one type of site - usually, a catalytically active metal site; these catalysts are monofunctional by virtue of lacking an acidic site for catalytic activity. Examples of monofunctional catalysts include the large pore zeolites, such as zeolites L, Y, and X and the naturally occurring faujasite and mordenite, wherein the exchangeable cation comprises a metal such such as alkali or alkaline earth metal; such catalysts also comprise one or more Group VIII metals providing the catalytically active metal sites, with platinum being a preferred Group VIII metal. Exchange of the metallic exchangeable cation of the zeolite crystal with hydrogen will provide acidic sites, thereby rendering the catalyst bifunctional.
A bifunctional catalyst is rendered bifunctional by virtue of also including acidic sites for catalytic reactions in addition to catalytically active metal sites. Included among conventional bifunctional reforming catalysts are those which comprise metal oxide support acidified by a halogen, such as chloride, and a Group VIII metal. A preferred metal oxide is alumina, and a preferred Group VIII metal is platinum.
The suitability of monofunctional and bifunctional catalysts for reforming varies according to the hydrocarbon number range of the fraction.
Both bifunctional and monofunctional catalysts are equally well suited for the naphthenes, or saturated cycloalkanes.
Monofunctional catalysts are particularly suited for reforming the C.sub.6 -C.sub.8 hydrocarbons. However, it has been discovered that the presence of dimethylbutanes, the lowest boiling of the C.sub.6 isomers, in the hydrocarbon fraction treated over monofunctional catalyst is commercially disadvantageous for two reasons.
As one reason, because of the reaction mechanism associated with monofunctional catalysts, they are not facile for dehydrocyclyzing dimethylbutanes to benzene. Instead, such catalysts crack a large portion of the dimethylbutanes to undesirable light gases.
As the second reason, dimethylbutanes have the highest octane rating among the non-aromatic C.sub.6 hydrocarbons, and are therefore of the most value in the mogas pool. Subjecting dimethylbutanes to catalytic activity renders them unavailable for upgrading the octane value of the mogas pool to the extent that they are cracked.
This discovery is the subject of a concurrently filed application, entitled PROCESS FOR REFORMING A DIMETHYLBUTANE-FREE HYDROCARBON FRACTION, 175570 this application is incorporated herein in its entirety by reference thereto.
It is known in the art to employ split feed reforming processes, wherein fractions of different hydrocarbon number range are separated out of a hydrocarbon feed, and subjected to different reforming catalysts. U.S. Pat. No. 4,594,145 discloses a process wherein a hydrocarbon feed is fractionated into a C.sub.5 - fraction, and a C.sub.6 + fraction; in turn, the C.sub.6 + fraction is fractionated into a C.sub.6 fraction and a C.sub.7 + fraction. The C.sub.7 + fraction is subjected to catalytic reforming, employing a catalyst most broadly disclosed as comprising platinum on an acidic alumina carrier. The C.sub.6 fraction is subjected to catalytic aromatization with a catalyst most broadly disclosed as comprising a Group VIII noble metal and a non-acidic carrier, with the preferred embodiment being platinum on potassium type L zeolite, which is monofunctional.
At column 3, lines 54-64, it is indicated that the C.sub.6 fraction advantageously contains at least 10 vol. % of C.sub.7 + hydrocarbons, with a general range of 10-50% by volume, and a preferable range of 15-35%. In Example 1, the C.sub.6 fraction is indicated to contain 3.2% C.sub.5 hydrocarbons, 72.7% C.sub.6 hydrocarbons, and 24.1% C.sub.7 + hydrocarbons. There is no disclosure or suggestion of limiting the proportion of C.sup.9 + hydrocarbons in the C.sub.6 fraction to less than 10% by volume of the fraction
As previously indicated, the monofunctional catalysts are particularly suited for reforming the C.sub.6 -C.sub.8 hydrocarbons, other than the dimethylbutane isomers. It has been discovered that the presence of more than about 10% by volume of C.sub.9 + hydrocarbons in the fraction treated with monofunctional catalyst will significantly inhibit catalytic activity.
In the process of this invention, the hydrocarbon fraction treated with monofunctional catalyst is limited to not more than about 10% by volume of C.sub.9 + hydrocarbons. This fraction preferably comprises not more than about 3%, and most preferably, not more than about 1% by volume C.sub.9 + hydrocarbons. The inventive process therefore provides benefits not taught by or disclosed in the prior art.
3. Definition of Terms
As used herein in the context of hydrocarbon or naphtha feeds, the terms "light fraction" and "heavy fraction" define the carbon number range of the hydrocarbons comprising the indicated fraction These terms are used in a relative manner; a "heavy fraction" is defined in reference to the carbon number range of its corresponding "light" fraction, and visa versa.
Specifically, a "light" fraction is a C.sub.6 fraction, a C.sub.7 fraction, a C.sub.8 fraction, a C.sub.6 -C.sub.7 fraction, a C.sub.7 -C.sub.8 fraction, a C.sub.6 -C.sub.8 fraction, or a fraction consisting essentially of C.sub.6 and C.sub.8 hydrocarbons; further, it is understood that, unless otherwise indicated, dimethylbutanes present in a light fraction amount to not more than about 10%, preferably about 3%, and, most preferably, no dimethylbutanes.
Further, a light fraction preferably comprises not more than about 10%, and, most preferably, not more than 2% by volume C.sub.5 - hydrocarbons. Of course, as discussed in detail herein, a light fraction also comprises, by volume, not more than about 10%, preferably not more than about 3%, more preferably, not more than about 1%, and, most preferably, no, or essentially no C.sub.9 + hydrocarbons.
C.sub.6 and C.sub.7 feeds will contain very little C.sub.9 content. It is the light fractions containing C.sub.8 hydrocarbons for which C.sub.9 + removal is critical
A "heavy" fraction comprises a range of hydrocarbons wherein the lowest carbon number compound is one carbon number higher than the highest carbon number compound of the corresponding light fraction.
Accordingly, when the light fraction is C.sub.6, the corresponding heavy fraction is C.sub.7 +. When the light fraction is C.sub.6 -C.sub.7 or C.sub.7, the corresponding heavy fraction is C.sub.8 +. When the light fraction is C.sub.8, C.sub.7 -C.sub.8, C.sub.6 -C.sub.8, or a fraction consisting essentially of C.sub.6 and C.sub.8 hydrocarbons, the corresponding heavy fraction is C.sub.9 +.
Unless specifically stated otherwise, the C.sub.5 - fraction is understood to include C.sub.6 dimethylbutane isomers. As stated above, the light fraction is understood essentially to exclude the C.sub.6 dimethylbutane isomers.
It is further understood that particular fractions are not necessarily comprised exclusively of hydrocarbons within the stated carbon number range of the fraction. Other hydrocarbons may also be present. Accordingly, a fraction of particular carbon number range may contain up to 15 percent by volume of hydrocarbons outside the designated hydrocarbon number range, subject to the limitation that the light fraction does not contain more than about 10% by volume of C.sub.9 + hydrocarbons.